Since mankind first collected more than it was possible to immediately consume, there has been a need for food containers to aid in preservation. In more modern times, food, flowers and other such materials have been stored in air-tight containers in that oxygen and moisture are often primary factors in food deterioration. Although the use of airtight containers can aid in slowing deterioration, foods and other related biological products are dynamically changing biological systems which are no longer growing but rather are deteriorating. Accordingly, when such a biological material is enclosed in a particular container, the environment is only maintained for a given period of time and that environment will change depending on the type of food and its original condition, temperature, environment, and other factors.
The storage of foods and other related biological materials can be broadly separated into two categories. In the first category there is included products which are not undergoing significant chemical/biological change. In the second category are materials which are undergoing significant chemical/biological changes. The changes relate to active metabolism and/or respiration of the materials. Examples of materials which are not undergoing significant change include cooked and canned foods, frozen foods and the like. It is generally desirable to keep such products in an oxygen free environment and to eliminate gas exchanges as much as possible to keep the products in acceptable condition. Products which are undergoing substantial change include products such as potatoes, strawberries, lettuce, meat, poultry, dairy products and the like. Such products are actively metabolizing and/or undergoing respiration and thereby require the benefit of an exchange of atmospheric gases including, but not limited to, carbon dioxide, oxygen, water vapor and other gases which may be given off by the materials such as various short chain hydrocarbons.
As an example of foods which could be sealed in an air-tight container and still deteriorate due to a change in environment, reference is made to fruits, which give off carbon dioxide during storage. A high concentration of carbon dioxide in the atmosphere in which fruits are stored causes a physiological breakdown of the fruit. To satisfactorily store fruit, it should be maintained at a relatively low temperature in an atmosphere of controlled carbon dioxide and oxygen concentration and the loss of moisture should be controlled within a given range.
The optimum storage conditions for any given biological material often varies between particular cultivars or varieties. Further, the optimum conditions for a given product may vary depending on the age, thermal past history and developmental stage of the material present within the container. Apples, for example, are advantageously stored at about 36.degree. F. and pears are stored at about 30.degree.-31.degree. F. For apples, the carbon dioxide content of the air within the storage container is preferably about 5-10% and for pears a concentration of from 5-20% or even as high as 25% is satisfactory. The rate of oxygen consumption and carbon dioxide generation by a given fruit varies with the age of the fruit and the temperature at which it is stored. Accordingly, it is difficult to maintain constant conditions within an air-tight storage container. Freshly picked apples give off carbon dioxide much more rapidly than apples which are three to four weeks old. If packaged immediately after picking, the carbon dioxide content of the air in the storage container during the first several weeks will be much higher than if the apples were stored at a later time. For this reason, earlier storage techniques often postpone packing the apples in containers until after the initial period of several weeks in which the carbon dioxide rapidly evolved. Attempts at solving the problem of carbon dioxide buildup were discussed at least as early as 1952 within U.S. Pat. No. 2,611,709, issued Sep. 23, -1952, to Homer H. Plagge. The Plagge disclosure teaches the use of different types of rubber hydrochloric films which have varying permeabilities with respect to certain gasses such as carbon dioxide due to the use of films with different thicknesses and films of different plasticizer content.
When packaging a completely different type of food, such as fresh, red meat, the oxygen content must be controlled within a narrow range in order to allow the meat to maintain the desirable red color. Complete lack of oxygen results in fresh meat having an undesirable purplish-red coloration. It is therefore desirable to use a packaging material which is at least partially permeable to oxygen. However, long continued oxidation of the myoglobin and oxymyoglobin results in the formation of brown discoloration. An earlier attempt at regulating the meat coloration by controlling the oxygen content is disclosed within U.S. Pat. No. 3,681,092 issued Aug. 1, 1972, to Titchenal et al., which discloses a method of packaging meat which includes enclosing the meat in a oxygen-permeable film and an oxygen-impermeable film and providing an evacuation port which allows for the control of gasses within the container.
Due to the irregular shapes of cut meats, the container must be structured so as to best eliminate puncturing of the food package. A specially designed meat package container is disclosed within U.S. Pat. No. 4,136,203, issued Jan. 23, 1979, to Murphy et al. incorporated herein by reference to disclose the construction of such packages. The Murphy et al. patent discloses placing the meat on a layer of foamed material and an oxygen impermeable surface. The meat is wrapped with a film so as to decrease the probability that punctures will be caused by the sharp edges of the meat.
Some of the problems with preserving foods can be alleviated by the use of refrigeration. However, some foods, such as tomatoes, are very sensitive to low temperatures and are physiologically injured if they are exposed to temperatures below 55.degree. F. The injury manifests itself in loss of flavor, a breakdown of cellular structure and, in later stages, rotting of the fruit. Attempts at maintaining the freshness of tomatoes without the use of refrigeration are disclosed within U.S. Pat. No. 4,079,152, issued Mar. 14, 1978, to Bedrosian et al. This patent discloses a gas-permeable film which allows the tomatoes to convert the package atmosphere to an environment containing a specific percentage of carbon dioxide and oxygen. The package contains chemical agents capable of absorbing moisture and carbon dioxide from the environment, which aid in preventing mold and deterioration of the tomatoes.
Another means of controlling the atmosphere within a food package is disclosed within U.S. Pat. No. 4,883,674, issued Nov. 28, 1989, to Fan. This patent discloses a package which includes a gas-permeable portion which allows a specific amount of oxygen into the package per unit of time. The method includes controlling the initial atmosphere within the package.
A number of different variations on airpermeable containers with and without the use of other chemicals which control the absorption of gases are disclosed within U.S. Pat. No. 3,706,410, issued Dec. 19, 1972, to Baker; U.S. Pat. No. 4,322,465, issued Mar. 30, 1982, to Webster; U.S. Pat. No. 4,657,610, issued Apr. 14, -1987, to Komatsu et al.; U.S. Pat. No. 4,856,650, issued Apr. 15, 1989, to Inoue.
Regardless of the type of package used and the gas-permeability of the material, the environment within the package changes due to chemical and biochemical reactions occurring within and around the food product present in the package. Accordingly, a number of oxygen scavenger compounds have been developed for use in connection with these packages. Examples of oxygen scavengers are disclosed within U.S. Pat. No. 4,299,719, issued Nov. 10, 1981, to Oaki et al.; U.S. Pat. No. 4,485,133, issued Nov. 27, 1984, to Ohtsuka et al.; and U.S. Pat. No. 4,536,409 to Farrell et al.
Each of the containers, materials and systems discussed above have advantages and disadvantages. When used in connection with certain foods, each can aid in preserving the color, quality and/or shelf life of a particular type of food product. However, they are generally limited to particular types of products, particular temperatures, and are more useful at certain points in the preservation of the food than at other points in time. As an example of the limitations of such packagings, it is pointed out that when food, such as plant tissue is sealed in a container, it tends to deteriorate due to the contact of the tissue with moisture condensed on the interior walls of the package container. This is especially troublesome in connection with plastic films and other relatively solid, moisture- and gas-impermeable materials, whether transparent or opaque. If the plant tissue is wrapped in close proximity or in contact with the interior wall of the wrapping material, the moisture comes in contact with the product and accelerates deterioration, and thus reduces the quality of the food product present in the package.
An elaborate method to aid in eliminating moisture buildup within the package is disclosed within U.S. Pat. No. 4,759,444, issued Jul. 26, 1988, to Barmore. This method uses gas-permeable surfaces and flushes certain gasses from the enclosure prior to sealing the package. However, the package is not a dynamic system. It is clear that, in order to provide a food package which aids in maintaining the color, quality and shelf-life of the food, the package must be somewhat dynamic. More specifically, as recognized by Theodore S. Lioutas in an article published in September 1988 in "Food Technology," there is a need for the development of "smart film" that could actually "sense" changes and adjust their permeability over a temperature range. The present invention provides for a food package which meets those needs by use of a polymer with a thermally responsive permeability.